Sticky Architecture: Encoding Pressure Sensitive Adhesion in Polymer Networks

Pressure sensitive adhesives (PSAs) are ubiquitous materials within a spectrum that span from office supplies to biomedical devices. Currently, the ability of PSAs to meet the needs of these diverse applications relies on trial-and-error mixing of assorted chemicals and polymers, which inherently entails property imprecision and variance over time due to component migration and leaching. Herein, we develop a precise additive-free PSA design platform that predictably leverages polymer network architecture to empower comprehensive control over adhesive performance. Utilizing the chemical universality of brush-like elastomers, we encode work of adhesion ranging 5 orders of magnitude with a single polymer chemistry by coordinating brush architectural parameters–side chain length and grafting density. Lessons from this design-by-architecture approach are essential for future implementation of AI machinery in molecular engineering of both cured and thermoplastic PSAs incorporated into everyday use.

( ) = 9 ( 2 − −1 ) (1 + 2 [1 − 3 ( 2 + 2 −1 )] −2 ) (S1) to the experimental stress-strain curve at a strain rate () along the elastic plateau. The output structural modulus, , and were subsequently used to calculate 0 from the derivative of Eq. S1 as → 1, 2 supporting time independent relaxation above . 3 Utilizing the Boltzmann superposition principle, stress evolution at small deformations during tensile testing of the PSA is Work of adhesion ( ) measurements. The ℎ is measured using a modified version of the probe tack test using a G2-RSA DMA. 6 The top arm contained a 2 mm diameter probe and the bottom a 25 mm plate with roughness of 0.5 microns (TA instruments). Segments of sample were placed on the bottom compression plate and allowed to wet the surface over time. In addition, a rubber roller was used to apply light pressure to ensure the adhesive bond between the elastomer and bottom plate (acting as carrier) remained intact during measurement. The run consisted of compression at 0.01 mm/s until a contact pressure, = 1 , was attained. The probe was held at a dwell time, = 100 and removed at 1mm/s for debonding.
Tensile hanging weight test. The hanging apparatus contained a curved steel loop attached to a level steel plane. A hook attached to a bottom pan was used to hold 10g weights and the hook was linked to the aforementioned steel bar. A 2mm sample was prepared by wiping down the surface with acetone, allowing any solvent to evaporate, and forming the adhesive bond by wetting one side of the adhesive to a steel horizontal wall and the other side of the PSA apparatus via the level steel plane. The sample was allowed to wet the surface for 100 seconds under pressure. The S4 apparatus without any weight was inverted so the pan was hanging and being upheld by the adhesive alone. The 10g weights were added sequentially until failure of the adhesive bond. The Tensile hanging stress was determined at the point of last weight addition.
Fused filament fabrication 3D printing. Fused-filament fabrication 3D printing was performed with a poly[nBA-ran-MMA-g-(PIB/PS)] thermoplastic elastomer sample using a Cellink BioX 3D printer where shape stl. files were created with Tinkercad in the shapes of biomedical adhesives (Figure 5d). The polymer reservoir was heated to 150℃ to ensure adequate flow and extruded at a pressure of 120 kPa.
Structural verification of brush structure by small-angle X-ray scattering (SAXS). 1 The SAXS measurements were carried out at the ID02 beamline of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. The experiments were conducted in transmission geometry using a photon energy of 12.46 keV. The recorded 2D data were centered, calibrated, regrouped and reduced to 1D using the SAXS utilities platform described elsewhere. 7 The analysis of the SAXS data was performed using the SANS data reduction and analysis package provided by NIST 8 for the Igor Pro environment (WaveMetrics Ltd.).
The monochromatic incident X-ray beam was collimated on the sample to a footprint of 100200 μm² (VH). The total photon flux was estimated to be 9.10 11 ph/s allowing for acquisition times of less than 100 ms. The accessed q values, with |q| = 4π.sin(ϴ)/λ, where ϴ is the Bragg angle and λwavelength, cover a range from 7.010 -3 nm -1 to 5.0 nm -1 . A Rayonix MX-170HS implemented in a 35m long vacuum flight tube was applied for recording of SAXS intensities at two different sample-to-detector distances of 1.5 and 10.0 m, respectively. For optimization of the scattering signal, a binning of 22 pixels was applied resulting in an effective pixel size of 89μm in both directions.

Synthesis and Characterization
Anti-markovnikov bromination of HR-PIB 1000. A 250 mL round bottom flask was prepared with a stir bar and 50 g (0.05 mmol) RB HR-PIB ( = 1000 / , Đ ~ 1.9) dissolved in hexane (150 mL) and placed in an ice bath. The solution was bubbled with air for 30 minutes at 0℃ and 24.3 g of 33 w/w% HBr (0.1 mol) in EtOAc was added dropwise to the flask with vigorous stirring. The solution reacted for 2 hrs at 0℃ followed by RT o/n. Stirring was ceased and the resultant anti-Markovnikov bromine functionalized PIB oligomer was washed with H2O/Na2CO3 twice (dried with anhydrous MgSO4) and extracted with a SiO2 column. The hexanes were evaporated by bubbling with air yielding 88% functionalized polymer (determined by H 1 -NMR, Figures S2-4). No residual olefin residue was present suggesting higher yield.

Synthesis of PIB (
= ) macromonomer. The functionalized oligomer was dissolved in THF (100 mL) and transferred to a clean 250 mL round bottom flask equipped with a stir bar. The solution was charged with 18.6 g KOMA (0.15 mol) and 48.3 g TBAB (0.15 mol) and ran 24 hrs at 45℃. The solution was centrifuged to remove residual salt and unreacted reagent. Subsequently, the solution was condensed by bubbling with air and washed with H2O/hexane twice. The organic layer was separated and ran through a SiO2 column revealing PIB ( = 18) macromonomer product (96% yield). Again, no residual peaks were present from the α-hydrogens suggesting higher yield. 1 H-NMR of the synthetic progression is shown in Figures S2-4. This synthesis applies to RB HR PIB-1300 ( = 23) and RB HR PIB-2300 ( = 41) with molar ratios respectively applied.
Synthesis of butyl acrylate by SARA ATRP. Poly(n-butyl acrylate) with different degree of polymerization were synthesized by supplemental activation reducing agent (SARA) atom transfer radical polymerization (ATRP) followed by a post polymerization functionalization displacing the bromine end group with potassium methacrylate. To a 500 mL air free Schlenk flask 120 g (0.94 ) of butyl acrylate was combined with Me6TREN (10 , 37 ), CuBr2 (8 mg, 36 ), and EBiB, 15.2, 7.3, or 3.7 g (0.078, 0.037, or 0.019 ) depending on desired , and diluted with an equal volume of acetonitrile. The reaction mixture was then cooled with an ice bath and oxygen was removed by bubbling nitrogen gas for 1 hour. The polymerization was initiated by adding a stir bar equipped with a clean Cu 0 wire and transferring to 45 C mineral oil bath. The reaction was monitored by 1 H NMR and stopped near 80% conversion with the addition of chloroform. Excess catalyst was removed by washing in water ~11 times and excess solvent was removed by rotary evaporation at 45 C under reduced pressure.
Synthesis of poly(n-butyl acrylate) macromonomers. The previously synthesized poly(n-butyl acrylate) was dissolved in 7 parts N,N-dimethylacetamide. Potassium methacrylate was added in large excess (>3 molar equivalents) and the reaction was left to stir for 3 days and turning a faint yellow color. To purify, the mixture was 1 part chloroform and 1 part water were added separating the mixture into two separate phases. The aqueous phase was discarded and the remaining organic S6 component was an addition 11 times with water until clear. Solvent was removed by rotary evaporation.
Synthesis of poly(n-butyl acrylate) macro-crosslinker by SARA ATRP. Poly(n-butyl acrylate) macro-crosslinkers were synthesized using an equivalent procedure to the poly(n-butyl acrylate) macromonomers. The one exception is that a difunctional 2-BiB ATRP initiator was used to polymerize n-BA such that the corresponding macromonomer was functionalized at both ends of the polymer chain. This difunctional poly(butyl acrylate) macro-crosslinker was also synthesized on a much smaller scale due to relatively small amount of it used during synthesis. The = 80 crosslinker was synthesized by combining 24 g (0.19 ) of butyl acrylate, Me6TREN (2 , 7.4 ), CuBr2 (1.6 mg, 7.2 ), and 2-BiB (0.67 g, 1.9 ) and diluting the mixture to 50% with acetonitrile. The reaction was then cooled in an ice bath and degassed for 1 hour with bubbling nitrogen gas. The polymerization was initiated by the addition of a Cu 0 wire and transferred to a 45 C oil bath until the reaction reached ~80% conversion. The reaction was then terminated by the addition of 50 mL of chloroform and washed 11 times in water. Solvent was removed by rotary evaporation at 45 C under reduced pressure. The cleaned polymer was then functionalized by the addition of 7 parts N,N-dimethylacetamide and a large excess of potassium methacrylate and left stirring for 72 hours. 50 mL of chloroform and 100 mL of water were then added separating the polymer into the organic phase. The organic phase was then washed in water 11 times until it became clear. Solvent was again removed by rotary evaporation at 45 C under reduced pressure.

Synthesis of PIB brush elastomer PSAs (Scheme 1).
A scintillation vial was charged with 5 g of PIB ( = 18) macromonomer (5 mmol), THF (5 mL), and R18 according to (ex. for = 1, = 100, R18 = 0.125 g). The vial was covered in aluminum foil and placed in an ice bath to prevent auto-initiation of R18. Furthermore, 7.0 mg (0.14 mol%) of BAPO was added to the vial. The vial was rapidly fixed with a rubber septum and was bubbled with nitrogen for 30 min. The deoxygenized solution was injected into a nitrogen flushed, hand-made glass mold and set to cure in a nitrogen chamber o/n (18-24hrs). The polymers were removed from the mold, swollen in THF twice to remove unreacted macromonomer (gel fraction > 90%). The PIB bottlebrush elastomers was dried o/n in fume hood followed by 2 hrs in in the oven at 60℃.

Synthesis of bottlebrush poly(n-butyl acrylate) elastomers (Scheme 2).
Bottlebrush poly(nbutyl acrylate) elastomers were synthesized by combining macromonomer (4 g), macrocrosslinker (1, 0.5, and 0.25 mol%), and BAPO (1.5 wt.%) were diluted to 50% in anisole. Nitrogen gas was used to purged with oxygen for 1 hour and then the mixture was injected into 1.3 mm thick elastomer molds and left to polymerize overnight in nitrogen atmosphere. The corresponding film was separated from its mold and a small portion was set aside to measure the samples corresponding gel fraction. The larger bulk part of the film was washed 3 times in toluene and dried prior to measurement. Gel fractions were for the most part at or above 90%. Gel fractions were measured by washing small sections of unwashed films in toluene 3 times over the course of 72 hours. The mass post washing divided by the mass of the gel fraction after washing was taken to be the gel fraction.

Synthesis of poly(butyl acrylate) comb elastomers (Scheme 2).
Poly(butyl acrylate) comb elastomers were synthesized by combining macromonomer, crosslinker (1, 0.5, and 0.25 mol%), BAPO (5-10 mg), and n-BA as spacer. The mixture was purged of oxygen using bubbling nitrogen gas and injected into a 1.3 mm molds and left to polymerize under ambient light conditions under a nitrogen atmosphere. As a specific example, for an [11,10,100] sample, [ , , ] 4 g of = 11 macromonomer, 3g n-BA spacer (9 molar equivalents), 0.0339 g (0.05 molar equivalents), and 5 mg of BAPO were used. A small portion of the film was removed to measure the gel fraction and the bulk part of the elastomer was washed 3 times in toluene and dried prior to sample measurement.

A-g-B brush copolymers (HMPSAs) 9
Synthesis of PS oligomers. ATRP of polystyrene homopolymer was performed with a target = 60 at 34% conversion to avoid large viscosities at higher conversion. Styrene (100g, 0.96mol), HEBIB (1.16g, 5.5mmol), PMDETA (0.095g, 0.114mL, 0.55mmol), and a stir bar were added to a Schlenk flask. The solution was bubbled with dry nitrogen for 1 hour then Cu(I)Br (0.079g, 0.55mmol) was quickly added to the reaction mixture under nitrogen atmosphere. The flask was sealed, purged for an additional 15 minutes, and then immersed in an oil bath at 90°C. The reaction mixture was left to polymerize for 14 hrs to receive a 34% conversion ( = 60) and the reaction was quenched by exposing the mixture to oxygen (Figures S8). The mixture was centrifuged and gravity filtered to remove residual Cu-ligand complex. Residual styrene monomer was evaporated and the remaining PS oligomer was dissolved in minimal THF and crashed in excess methanol (1:10, THF:Methanol by volume) 3 times. The washed PS oligomer was dried overnight at room temperature under reduced pressure to remove any residual solvent. The PS oligomer (30 g, 4.8mmol, DP = 60) was transferred to a round bottom flask sealed by rubber septum and parafilm and dissolved in 60 ml of THF. Once the PS was fully dissolved, 0.05g (47μL, 80μmol) dibutyltin dilaurate was added to the solution, it was subsequently purged of oxygen by bubbling the solution with nitrogen for 10 minutes. IEM (0.82g, 0.75mL, 5.3mmol) was added dropwise to the round bottom flask under constant stirring. Nitrogen was removed from the flask, and the solution was set to stir for 18hr. The subsequent solution was further diluted with THF (5-10x) and passed through silica column twice. The purified mixture was dried under reduced pressure and characterized by 1 H-NMR ( Figure S8). H 1 -NMR reveals 80% conversion so subsequent calculations for for performed considering an 80% ratio of macromonomers.             . nBA and PIB macromonomer were polymerized in accordance with = 4 (4 mol:1 mol, respectively) by atom transfer radical polymerization (ATRP) to study the addition of monomer over 20 hours. The polymerization of brush copolymer was monitored by consumption of PIB macromonomer and nBA monomer. This is understood as an analog to our free-radical UV-curing methodology and concede potential effects of catalyst and ligand interactions in ATRP.

Mechanical properties
=0.001s -1 , T=20℃. A-g-B brush copolymer are much stronger than the UV-cured elastomers. b) Frequency sweep of a sample A-g-B brush copolymer. c) Chang window for a sample A-g-B brush copolymer. The window shifts to the upper right indicating potential use as a high shear PSA.  (1) Grafting density of side chains on the backbone with BA spacer. (2) Number average degree polymerization of brush backbone between glassy block side chains that physical crosslink. (3) Number average degree polymerization of each glassy block side chain as determined by 1 H-NMR. (4) Volume fraction glassy block, = 0.92 ⁄ , = 1.02 ⁄ , = 1.08 ⁄ . (5) Number average degree polymerization of the total brush strand. (6) Structural modulus ~1 ( ( + 1)) ⁄ and (7) strain-stiffening parameter = 〈 2 〉 2 ⁄ are fitting parameters in equation S1. (8) Apparent Young's modulus determined either as tangent of a stress-strain curve at →1 or from the fitting equation S2. (9) Elongation range used for fitting equation S1 before deviation from the theory. (10) Maximum true stress and elongation at sample rupture. (11) Maximum stress-at-break (strength) of A-g-B brush copolymer samples. Figure S19. PIB viscoelastic control with . As increases, energy dissipation increases disproportionately resulting in greater tanδ. All bottlebrush PIB samples witness the proper balance between energy storage and dissipation with tanδ~1 within the frequency of the Chang window. T=20℃. Figure S20. PIB viscoelastic control with . As increases, energy dissipation decreases disproportionately resulting in lower tanδ. Samples with greater grafting density of PIB macromonomer (i.e. = 1,2) witness the proper balance between energy storage and dissipation with tanδ~1 within the frequency of the Chang window. T=20℃. Figure S21. PIB viscoelastic control with . As increases, energy dissipation increases disproportionately resulting in greater tan δ. T=20℃. Figure S22. PBA viscoelastic control with . As increases, energy dissipation increases disproportionately resulting in greater tanδ. All bottlebrush PBA samples witness the proper balance between energy storage and dissipation with tanδ~1 within the frequency of the Chang window. T=20℃. Figure S23. PBA viscoelastic control with . As increases, energy dissipation decreases disproportionately resulting in lower tanδ. Samples with greater grafting density of PBA macromonomer (i.e. = 1,2,3) witness the proper balance between energy storage and dissipation with tanδ~1 within the frequency of the Chang window. T=20℃. Figure S24. PBA viscoelastic control with . As increases, energy dissipation increases disproportionately resulting in greater tanδ. Brush PBA elastomer PSA samples witness the proper balance between energy storage and dissipation with tanδ~1 within the frequency of the Chang window. T=20℃. Figure S25. Example of Rouse time derivations from uniaxial tensile testing at various strain rates. a) Stress-elongation curves at various strain rates for a sample PBA brush PSA. b) Determining Rouse time from the slope-transition of rate normalized stress defining deformation in the Rouse regime (slope=0.5) to deformation on the elastic plateau (slope=1). T=20℃. Figure S26. Stain rate normalized stress dependence on time for PIB brush PSA samples. The plots show a slope transition from 0.5 to 1 as the deformation of the sample transitions from the Rouse regime where the network can be approximated as a melt of polymer strands, to the elastic regime where contributions of crosslinks and entanglements must be considered. The of PIB brush elastomer PSAs was verified by differentiation and creation of a master curve in which all lines converge (Figures S28,29). T=20℃. 10mm/s (black), 1 mm/s (red), 0.1 mm/s (blue), 0.01 mm/s (green), 0.001 mm/s (purple), 0.0001 mm/s (yellow). Linear rates were translated to strain rates based on the initial length of the sample in the clamps. Figure S27. Stain rate normalized stress dependence on time for PBA brush PSA samples. The plots show a slope transition from 0.5 to 1 as the deformation of the sample transitions from the Rouse regime where the network can be approximated as a melt of polymer strands, to the elastic regime where contributions of crosslinks and entanglements must be considered. The of PBA brush elastomer PSAs was verified by differentiation and creation of a master curve in which all lines converge (Figures S28,29). T=20℃. 10mm/s (black), 1 mm/s (red), 0.1 mm/s (blue), 0.01 mm/s (green), 0.001 mm/s (purple), 0.0001 mm/s (yellow). Linear rates were translated to strain rates based on the initial length of the sample in the clamps.    (Table S4), where error bars are displayed in red on the plot to the right. PBA [11,1,200]. T=20℃. Table S4. Precision of modified probe tack test with strain rate. Error was determined by standard deviation of triplicate measurements from Figure S31. (1) Strain rate of debonding during the probe tack test. (2) Overall average work of adhesion for a desired debonding rate with relative uncertainty of a triplet of trials determined by standard deviation. (3) Representative present error for a given strain rate in determining ℎ . The percent error within the debonding strain rate range of 0.001-10 s -1 remains < 4% for the modified probe tack test used for all samples. Figure S32. a) Brush PSAs display greater work of adhesion than a commercial ostomy bag and b) adhesion does not change after extraction in toluene and after exposure to high temperatures for a sample PBA brush PSA. T=20℃. Figure S33. Raw modified probe tack test spectra for PIB brush PSAs. All PIB brush elastomer PSAs were subjected to modified probe tack testing at variable strain rates. As you increase , fibril elongation before catastrophic failure of the adhesive bond increases though the stress from the fibrils decreased. Increasing the grafting density of the PIB side chains also increased the tack and fibril elongation. With increased , fibrillar stress increased and the fibril elongation increases minimally. T=20℃. Note: some samples were measured from = 10 − 0.01 / rather than = 1 − 0.001 / , please see Table S5 to verify.  (2) Thickness normalized strain rate of debonding. (3) The overall work of adhesion determined from Eq. 2. Figure S34. Raw modified probe tack test spectra for PBA brush PSAs. All PBA brush elastomer PSAs were subjected to modified probe tack testing at variable strain rates. As you increase , fibril elongation before catastrophic failure of the adhesive bond and tack increase. Increasing the grafting density of the PBA side chains also increased the tack and fibril elongation. With increased , fibril elongation increases and tack peaks manifest. Note: all samples were measured from = 1 − 0.001 / , 1mm/s (black), 0.1 mm/s (red), 0.01 mm/s (blue), 0.001 mm/s (green). (1) Linear velocity of debonding during the probe tack test of PBA brush elastomer PSAs. (2) Thickness normalized strain rate of debonding. (3) The overall work of adhesion determined from Eq. 2.    (Table S1). Their rheological curves also vary greatly within the Chang window. However, the work of adhesion is nearly identical through four decades of strain rate. This can be attributed to different modes of debonding and resistance to deformation as well as the difference in thermodynamic work of adhesion between the adhesive and probe. T=20℃.

Theoretical Analysis Rouse time of brush elastomer PSA's.
To elucidate the dependence of the Rouse time of bottlebrush polymers on grafting density and degree of polymerization of the side chains, we assume that two bottlebrushes contact each other at the surface of their perspective side chain blobs due to expulsion of the side chains and backbones. This results in the macromolecular friction coefficient being proportional to the number of monomers on the outer surface of bottlebrushes. The surface area of the bottlebrush ( Figure S38) with backbone degree of polymerization is estimated as is the side chain size and is the number of backbone monomers per side chain thickness. Taking into account the packing condition of monomers having excluded volume (1 + / ) / 3 ≈ 1 → 2 / ≈ −1 / (S8) the total number of monomers on the surface of the brush is ≈ / 2/3 ≈ 1/3 −1 / . (S9) The corresponding friction coefficient of the bottlebrush with monomer friction coefficient 0 is equal to = 0 ≈ 0 1/3 −1 / (S10) The Rouse time of the brush macromolecule with size 2 ≈ 2 / ≈ −1 / (S11) is estimated as ≈ 2 / ≈ 0 4/3 ( −1 ) 2 / 2 (S12) For brushes with stretched backbones and ideal side chains ( 2 ≈ ) corresponding to the SBB regime, eq S12 is rewritten as follows ≈ 0 ( −1 ) 2 / (S13) where 0 ≈ 0 4/3 / is a characteristic relaxation time of brushes with monomer projection length and Kuhn length . Note that a peculiar dependence of 0 on polymer chemistry specific parameters , and is due to brush friction mechanisms and side chain packing constraints. It is different from that of linear chains in a melt for which a characteristic relaxation time 0, ≈ 0 / . For brush strands with densely grafted side chains −1 ≈ / , we obtain the following dependence of the Rouse time on brush architecture (main text, Eq. 1), (S14) It is important to point out that we will have a different dependence of the Rouse time on the molecular architecture if brush friction is determined by all monomers. In this case, the net friction coefficient is equal to = 0 −1 (S15) and the corresponding Rouse time is ≈ 2 / ≈ 0 ( −1 ) 2 / (S16) Figure S38. A brush macromolecule as a chain of blobs with size each containing backbone monomers.